1) Field of the Invention
The present invention relates to a method of and an apparatus for controlling the oscillation wavelength of a wavelength variable semiconductor laser. The wavelength variable semiconductor laser is used in wavelength division multiplex transmission (WDM).
2) Description of the Related Art
In an optical communication system that utilizes an optical fiber, a high-density wavelength division multiplexing (DWDM) system has come to be used. This DWDM system is a system that transmits waves by multiplexing a plurality of different wavelengths in one optical fiber. It is necessary to stabilize the light wavelengths with high precision.
A distributed feedback (DFB) laser is generally used as a light source in the DWDM system. This DFB laser forms a diffraction grating that selectively reflects a wave having only one wavelength, in an optical amplification area. Therefore, the use of the DFB laser stabilizes the oscillation-mode and can realize a semiconductor laser producing single wavelength light. Usually, one light source is used for one channel (wavelength) in an optical machine that is used in the WDM system. As the DFB laser has a small wavelength variable area, one DFB laser is necessary for one channel in a preparatory light source for handling trouble. This makes the system expensive.
To make the system cheaper, it is necessary to use a laser having a large wavelength variable area for the light source. A distributed Bragg reflector (DBR) semiconductor laser has diffraction gratings disposed at both sides of an optical amplification area as light reflectors having wavelength dependency. This DBR semiconductor laser selectively reflects a wave having only a specific wavelength, and amplifies the wave in the optical amplification area, thereby generating an oscillation light having one peak wavelength. It is possible to change the oscillation wavelength by about a few dozen nm by changing the injection current to the diffraction gratings at both sides. However, in the semiconductor laser having a DBR structure with relatively small intervals between the reflection peaks, the oscillation-mode becomes unstable. This unstable oscillation-mode occurs due to mode hopping in which a wavelength jumps to an adjacent wavelength when the reflection peaks of the diffraction gratings at both sides coincide with each other, or due to contention between adjacent oscillation longitudinal-modes.
As methods of controlling the wavelength of the wavelength variable semiconductor laser, there is a current control system for controlling the injection current to a light reflector, and a temperature adjusting system for adjusting the temperature of the laser device.
Control methods using a current control table as one of the current control systems are disclosed in xe2x80x9cA High-speed mixed Digital-to-Analog circuit board for accurate control of wavelength tunable lasers for fiber-opticxe2x80x9d Paul D. Biernacki, et al., Journal of Lightwave Technology. vol. 17, No. 7, July (1999) 1222-1228. Or xe2x80x9cControl of widely tunable SSG-DBR Lasers for dense wavelength division multiplexingxe2x80x9d Gert. Sarlet, et al. Journal of Lightwave Technology. vol. 18 No. 8 (2000) 1128-1138. Or xe2x80x9cMode stabilization method for Superstructure-Grating DBR Lasersxe2x80x9d Hiroyuki. Ishii, et al., Journal of Lightwave Technology, vol. 16 No3(1998) 433-442.
According to the above control methods using a current control table, a two-dimensional data table is prepared that has injection currents I1 and I2 to the light reflectors (diffraction gratings) at both sides as variables. Data relating to the oscillation characteristic is registered for each combination of the injection currents I1 and I2. The wavelength is controlled using the registered data. According to the control method using the current table system, response follow-up of the wavelength control is satisfactory, and it is possible to achieve high-speed control. However, according to this control method using the current table system, control of the laser device with the lapse of time is not satisfactory. Further, it is essential to prepare a current table that prescribes the oscillation wavelength of each laser device for inspection prior to the shipment. This results in an increase in cost.
On the other hand, according to the method based on temperature control, the stability of the control of the prescribed wavelength is excellent. However, a time of milliseconds is necessary to stabilize the temperature, and it is not possible to carry out high-speed control. Further, when the temperature of the laser device has changed, an optimum point of wavelength control changes due to the temperature dependency of the internal quantum efficiency and the gain curve. Consequently, control becomes complex. Further, according to the method based on temperature control, the wavelength range in which the wavelength can be adjusted within a practical temperature range is about only a few nm.
As a conventional technique of a hybrid system that takes into account the advantages of both methods, there is one method as disclosed in Japanese Patent Application Laid-open No. 9-931107. According to this conventional technique, first, the wavelength is shifted based on current control. Then, the wavelength shift based on the current control is replaced with a wavelength shift quantity based on the temperature control. With this arrangement, the wavelength control is carried out taking into account both advantages of the current control in which response is fast but the wavelength variable range is narrow and the temperature control in which the wavelength variable range is wide but response is slow.
According to this conventional technique, both high speed and stability are realized by combining the current control and the temperature control. However, the time lapse of the laser device is managed based on the temperature control. Therefore, the operation condition changes, and the control becomes complex. According to this conventional technique, it is still essential to prepare the current table for inspection prior to the shipment.
The above conventional techniques have problems in that it is expensive to prepare a current table for each device prior to shipment, and that it is difficult to guarantee long-term stability.
According to the conventional technique of the hybrid system, the long-term stability and the current setting method have been improved to some extent. However, as this control method is based on the reflection mode of each mirror, the range of stability is small. In other words, it is not possible to simplify the inspection to correspond to a constant-output operation including mirror loss. It is not possible to manage time lapse exceeding a wavelength mode boundary. Further, as a modulation-synchronized detection is used for a signal detection method, the module installation becomes complex and expensive.
It is an object of this invention to provide a method and apparatus that can converge an oscillation control point to a stable point corresponding to a target wavelength in a more efficient manner.
The wavelength control apparatus according to one aspect of the present invention controls an oscillation wavelength of a wavelength variable semiconductor laser. The wavelength variable semiconductor laser has first and second light reflectors and an active layer area disposed between the first and second light reflectors. Each of the first and second light reflectors has a plurality of reflection peaks. An oscillation state deciding section decides the oscillation state of the wavelength variable semiconductor laser. A drive controller obtains the current conditions of an injection current to the first light reflector and an injection current to the second light reflector that carry out single-mode oscillation in a target wavelength, by simultaneously sweeping the injection current to the first light reflector and the injection current to the second light reflector based on the oscillation status obtained by the oscillation state deciding section. Moreover, the drive controller drive-controls the wavelength variable semiconductor laser with the obtained current conditions. As a result, there is no need to prepare a high-precision current table.
The wavelength control method according to another aspect of the present invention controls an oscillation wavelength of a wavelength variable semiconductor laser. The wavelength variable semiconductor laser has first and second light reflectors and an active layer area disposed between the first and second light reflectors. Each of the first and second light reflectors has a plurality of reflection peaks. In changing the oscillation wavelength, a same-oscillation-mode area that is encircled by two oscillation-mode boundaries that includes the oscillation point corresponding to the current wavelength to thereby obtain a virtual point corresponding to the target wavelength is extended. Moreover, the oscillation control point corresponding to the changed wavelength beyond the oscillation-mode boundary by using predetermined equations based on the injection currents to the first and second light reflectors and the oscillation-mode boundaries for this virtual point and the oscillation point corresponding to the current wavelength is obtained. As a result, there is no need to prepare a high-precision current table.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.